It is well known that hydrocarbons (oil and gas) are produced from wells drilled in the earth, hereinafter referred to as “oil wells.” It is additionally well known that drilling a hole into the earth to reach oil and gas bearing formations is an expensive operation which limits the number of wells that can be economically drilled. It follows then that it is desirable to maximize both the overall recovery of hydrocarbons held in the formation and the rate of flow from the subsurface formation to the surface, where it can be recovered.
One way in which to maximize production is the process known as hydraulic fracturing. Hydraulic fracturing involves cracking or fracturing a portion of the hydrocarbon-bearing formation surrounding an oil well by injecting a specialized fluid into the wellbore directed at the face of the geologic formation at pressures sufficient to initiate and/or extend a fracture in the formation. Ideally, what this process creates is not a single fracture, but a fracture zone, that is, a complex zone having multiple fractures, or cracks in the formation, through which hydrocarbon can more readily flow to the wellbore. Proppants and other materials are pumped into the fractures or cracks to keep the fracture open once the hydraulic pressure is released. Such propped fractures have increased permeability compared to the surrounding rock, which improved permeability facilitates the production of hydrocarbons. The proppants or other materials may contain substrate particles such as diatomaceous earth (DE) that may have treating materials adsorbed or otherwise contained on the substrate. These treating materials may include, but not necessarily be limited to, scale inhibitors, paraffin inhibitors, corrosion inhibitors, and the like, which may desorb from the substrate particles to treat the produced hydrocarbons.
Fracturing fluids can vary widely in composition. Slick water is water to which has been added chemicals to increase its fluid flow, notably friction reducers such as a polyacrylamide. Friction reducers improve the ability of the fluid to be pumped under pressure to cause fracturing and with less power than essentially only water. Other optional components include biocides, corrosion inhibitors, scale inhibitors and the like. Fracturing fluids may also comprise water that has been viscosified, such as by using a crosslinked or non-crosslinked polysaccharide such as guar gum or the like, and/or by using a viscoelastic surfactant (VES) such as an amidoamine oxide.
Creating a fracture in a hydrocarbon-bearing formation requires several materials. Often these materials, if not removed from the oil well, can subsequently interfere with oil and gas production. Even the drilling mud used to lubricate a drill bit during the drilling of an oil well can interfere with oil and gas production. Taking too long to remove such materials can increase the cost to the operator of the well by delaying production and causing excess removal expenses. Not being thorough in removing such materials can increase the cost to the operator of the well through lower production rates and possible lost production.
Measures taken to remove unwanted or unneeded materials are usually inexact. Sometimes additional fluids are used to flush out unwanted materials in the well bore. In other situations, reservoir fluids flow can make estimating return flow very difficult, particularly if the reservoir fluids are incompatible with the injected materials. It would be desirable in the art of oil and gas production to be able to determine how much of a given material is left in an oil well after a drilling, fracturing or any other operation requiring the injection of materials into an oil well. Tracers included in the material are a known way of determining the presence, and sometimes the amount, of a given material remaining in or retrieved from an oil well with which the tracers are associated.
One hydraulic fracturing technique uses multiple fracturing stages where different isolated zones are fractured in different ways designed or customized for each zone. However, once the well is placed into production, and the fluids from all zones are produced together, it generally cannot be determined from which zone a particular portion of the fluid was produced since the fluids are intermingled. In the past, unique halogenated tracers have been injected into each of the respective zones, and by means of distinguishing the produced tracers and their associated fluids, it may be determined what types of fluids (and their compositions) are produced from which zones. Further, the tracers may help maximize the production of oil and gas. If it is determined that water is overwhelmingly produced from one particular zone, that zone could be isolated and shut off from production so that less overall water is produced and the hydrocarbon production may be maximized.
In the past, perfluorinated compounds have been used as tracers to measure oil returns from a fracturing job. These compounds have a one particular key advantage and many disadvantages. Their main advantage is they are easy to detect. However, they are very expensive, and further, separating one compound from another is very difficult. Also, halogenated compounds remaining in the produced fluids will poison the catalyst in the downstream refineries.
It would be particularly desirable if these goals could be achieved using inexpensive tracers which are easily distinguished from one another.